From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8A7F3C282C0 for ; Wed, 23 Jan 2019 09:34:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E21B21019 for ; Wed, 23 Jan 2019 09:34:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727100AbfAWJed (ORCPT ); Wed, 23 Jan 2019 04:34:33 -0500 Received: from foss.arm.com ([217.140.101.70]:38106 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726207AbfAWJed (ORCPT ); Wed, 23 Jan 2019 04:34:33 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AB60DA78; Wed, 23 Jan 2019 01:34:32 -0800 (PST) Received: from brain-police (usa-sjc-mx-foss1.foss.arm.com [217.140.101.70]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BA2173F5C1; Wed, 23 Jan 2019 01:34:29 -0800 (PST) Date: Wed, 23 Jan 2019 09:34:25 +0000 From: Will Deacon To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Borislav Petkov , "H. Peter Anvin" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, x86@kernel.org, Zhenzhong Duan , James Morse , SRINIVAS Subject: Re: [PATCH v2 1/4] locking/qspinlock: Handle > 4 slowpath nesting levels Message-ID: <20190123093424.GE15019@brain-police> References: <1548215351-18896-1-git-send-email-longman@redhat.com> <1548215351-18896-2-git-send-email-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1548215351-18896-2-git-send-email-longman@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 22, 2019 at 10:49:08PM -0500, Waiman Long wrote: > Four queue nodes per cpu are allocated to enable up to 4 nesting levels > using the per-cpu nodes. Nested NMIs are possible in some architectures. > Still it is very unlikely that we will ever hit more than 4 nested > levels with contention in the slowpath. > > When that rare condition happens, however, it is likely that the system > will hang or crash shortly after that. It is not good and we need to > handle this exception case. > > This is done by spinning directly on the lock using repeated trylock. > This alternative code path should only be used when there is nested > NMIs. Assuming that the locks used by those NMI handlers will not be > heavily contended, a simple TAS locking should work out. > > Suggested-by: Peter Zijlstra > Signed-off-by: Waiman Long > --- > kernel/locking/qspinlock.c | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c > index 8a8c3c2..0875053 100644 > --- a/kernel/locking/qspinlock.c > +++ b/kernel/locking/qspinlock.c > @@ -412,6 +412,21 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) > idx = node->count++; > tail = encode_tail(smp_processor_id(), idx); Does the compiler generate better code if we move the tail assignment further down, closer to the xchg_tail() call? > + /* > + * 4 nodes are allocated based on the assumption that there will > + * not be nested NMIs taking spinlocks. That may not be true in > + * some architectures even though the chance of needing more than > + * 4 nodes will still be extremely unlikely. When that happens, > + * we fall back to spinning on the lock directly without using > + * any MCS node. This is not the most elegant solution, but is > + * simple enough. > + */ > + if (unlikely(idx >= MAX_NODES)) { > + while (!queued_spin_trylock(lock)) > + cpu_relax(); > + goto release; > + } Acked-by: Will Deacon Will